Multi-layer systems have been utilized to coat automobiles for a number of years, but the early development of these systems necessarily employed organic solvents. As environmental regulations became more stringent organic-borne systems became less desirable. The recent research emphasis in the area of multi-layer systems, especially basecoat systems, has focused on the development of water-borne systems for multi-layer coatings.
In particular, as organic-borne systems were formulated to require less and less organic solvent, becoming known in the industry as "high solids" coatings, it became obvious that the appearance, in particular the metallic effect of coatings containing metallic pigments, suffered with the increase in solids. One of the opportunities which water-borne systems presents is that of improving on the metallic effect in a coating which complies with the restrictions on volatile organic content. For example, the U.S. Pat. Nos. 4,880,867 and 4,954,559 discloses an aqueous coating composition comprising a mixture of an acrylic dispersion and a polyurethane dispersion, which can be used as a primer or a basecoat. Water-borne systems, however, resulted in other problems.
When waterborne coatings, which contain aluminum pigments are applied over steel substrates, scribed through the coating to bare steel, and exposed to a sodium chloride salt spray (ASTM B-117), the metallic effect disappears, leaving either the solid color of the coating or of the primer, if the coating contains no other pigments than the aluminum. This phenomenon is the result of the sodium chloride solution functioning as an electrolyte in the presence of the two dissimilar metals, e.g., the aluminum pigment and the steel substrate. The phenomenon is described is termed galvanic action.
The object of the present invention was to provide an aqueous coating composition as a waterborne primer for metallic substrates which does not show the above mentioned disappearance of the metallic effect. Another object was to provide an aqueous primer with a low volatile organic content, a method of coating a substrate with said primer and a substrate coated with at least said primer.